创建EntityManagerFactory时，可以传递一组属性，这些属性将覆盖persistence.xml中定义的属性，例如:

When you create the EntityManagerFactory, you can pass a set of properties that will override what is defined in persistence.xml, e.g.:

Properties props = new Properties();
props.setProperty("javax.persistence.jdbc.url", "test_url");
_emf = Persistence.createEntityManagerFactory("dev", props);

如果要在创建EntityManagerFactory之后修改连接属性，则必须通过再次调用createEntityManagerFactory()重新创建它.

If you want to modify the connection properties after the EntityManagerFactory was created, you must recreate it by calling createEntityManagerFactory() again.